1. Field of tile Invention
This invention relates to a focus detecting device for use in cameras.
2. Description of the Related Art
In the past, many focus detecting optical systems have been proposed, each of which is such that an image formed by a photographic lens is divided into two, symmetrical with respect to a plane including its optical axis, by a reimaging optical system, which are re-formed on photoelectric converting element arrays (light receiving element arrays), and the positional shift between these two images is detected to thereby perform focus detection. These proposed devices are set forth, for example, in Japanese Patent Preliminary Publication Nos. Sho 55-118019, Sho 58-106511, and Sho 60-32012. Any of them is designed so that the amount of light through the reimaging optical system is received by the light receiving element arrays in a line, and their output signals are used for focus detection.
In the focus detecting (optical) system, the light receiving element array is such that light receiving elements are in general arrayed at regular intervals. When the distance between two light receiving elements adjacent to each other is taken as one pitch, focusing accuracy is represented as a relative measure for one pitch. Now, when focusing accuracy is assumed to be 1/M (where M is a constant) of one pitch and the amount of defocus per pitch at the image plane is denoted by .alpha., a focusing accuracy .DELTA. at the image plane is defined as EQU .DELTA.=.+-.(1/M) .alpha.(tm) (1)
Here, focusing accuracy degrades with increasing value .DELTA. and becomes high with decreasing it. In other words, the greater the value of the constant M, the higher the focusing accuracy. The constant M is determined by the coincidence of two images being compared on the light receiving element arrays and the accuracy of calculation for detecting the correlation (the amount of phase shift). The increase of the number of light receiving elements used to detect the correlation (the amount of phase shift) results in improved calculation accuracy. In a camera system requiring the constant M of 10 or more, for example, when the phase difference is detected in correlation with the output corresponding to 20 elements, it is required that the coincidence of the images on the light receiving clement arrays is excellent compared with the case of 0.1 times the pitch on the 20 elements (the length 20 times the pitch), provided the calculation accuracy is sufficient. This is 0.005 times the pitch in terms of one element.
When the number of light receiving elements of the tight receiving element array is taken as N (a constant), a detectable defocus area .SIGMA. (which is hereinafter referred to as a focus detecting area .SIGMA.) at the image plane is expressed by EQU .SIGMA.=.vertline..+-.N .alpha..vertline. (2)
In Eqs. (1) and (2), if the amount of defocus is increased, the focus detecting area .SIGMA. will be large but the focusing accuracy .DELTA. will be poor. Conversely, if the amount of defocus .alpha. is decreased, the focusing accuracy .DELTA. will be improved but the focus detecting area .SIGMA. will be diminished. Thus, the focus detecting area .SIGMA. and the focusing accuracy .DELTA. are affected, opposite to each other, by the amount of defocus .alpha. per pitch at the image plane. It follows from this that the extension of the focus detecting area .SIGMA. and the improvement of the focusing accuracy .DELTA. are incompatible with each other.
In order to solve the above problem, the use of a plurality of focus detecting systems is proposed by U.S. Pat. No. 4,959,677 and Japanese Patent Preliminary Publication Nos. Sho 63-264715 and Hei 4-240813. Referring now to FIGS. 1 to 7, the proposal of U.S. Pat. No. 4,959,677 is explained in some detail. FIG. 1 shows the case where a focus detecting device is situated on the bottom of the body of a single-lens reflex camera. FIG. 2 shows focus detecting systems orthogonal to each other. FIG. 3 depicts essential parts of one of the focus detecting systems. FIG. 4 depicts essential parts of the other, which is rotated 90.degree. about the optical axis with respect to FIG. 3.
In these diagrams, reference numeral 1 represents a photographic lens; 2 a preset imaging plane; 3 a condenser lens disposed adjacent to the preset imaging plane 2; and 4 an aperture stop. The aperture stop 4 is located behind the condenser lens 3 and includes openings, four in total, arranged in directions perpendicular to each other, one pair in each, with a space sufficient to ensure focusing accuracy. Reference numeral 5 denotes a separator lens. The separator lens 5 is situated behind the aperture stop 4 with two pairs of openings and includes reimaging lenses, four in total, arranged in directions perpendicular to each other, one pair in each, to correspond to individual openings. Numerals 6 and 7 denote two pairs of light receiving element arrays disposed in directions normal to each other. The light receiving element arrays 6 and 7 are located at the imaging positions of light beams emerging from the separator lens 5.
FIGS. 5, 6, and 7 show the aperture stop 4, the separator lens 5, and the light receiving element arrays 6 and 7, respectively, viewed along the direction of the optical axis. The light beams passing through individual openings of the aperture stop 4 are independent of one another. The optical components described above constitute the focus detecting device.
In the focus detecting device of U.S. Pat. No. 4,959,677, the focus detecting systems perpendicular to each other are respectively taken as a focus detecting system I and a focus detecting system II. The amount of defocus D.sub.1 detected by the focus detecting system I and the amount of defocus D.sub.2 detected by the focus detecting system II are given by EQU D.sub.1 =(F.sub.W1 /.beta.) P.sub.1 ( 3) EQU D.sub.2 =(F.sub.W2 /.beta.) P.sub.2 ( 4)
where P.sub.1 is the amount of phase difference of the images on the light receiving clement arrays of the focus detecting system I, P.sub.2 is the amount of phase difference of the images on the light receiving element arrays of the focus detecting system II, .beta. is the imaging magnification of the focus detecting systems I and II, F.sub.W1 is the F number of the barycentric beam to be detected in the focus detecting system I, and F.sub.W2 is the F number of the barycentric beam to be detected in the focus detecting system II. Here, the term "barycentric beam" means the light beam defined by rays passing through the center of each opening of the aperture stop. Thus, proper settings of the F numbers F.sub.W1 and F.sub.W2 and the magnification .beta. of the focus detecting systems I and II allow the constructions of the focus detecting system I which is somewhat low in focusing accuracy but large in focus detecting area, and the focus detecting system II which is smaller in focus detecting area but higher in focusing accuracy. In this way, the focus detecting device can be derived in which the extension of the focus detecting area and the improvement of the focusing accuracy are compatible with each other.
Using FIG. 8, reference is made to the proposal of Japanese Publication No. Sho 63-264715. FIG. 8 shows the focus detecting device in which two focus detecting systems are juxtaposed which involve a TTL phase difference technique set forth in this publication. There are the condenser lens 3 disposed adjacent to the preset imaging plane 2, and a half mirror 8 and a reflecting mirror 9 which are situated behind the condenser lens 3. On the optical path reflected from the half mirror 8 are arranged an aperture stop 4a having a pair of openings juxtaposed, normal to the plane of the page, at a distance sufficient to ensure focusing accuracy; a pair of separator lenses 5a disposed behind the aperture stop 4a having a pair of openings; and light receiving element arrays 6 placed at imaging positions of light beams emerging from the separator lenses 5a. On the optical path transmitted through the half mirror 8 and reflected from the reflecting mirror 9, on the other hand are arranged an aperture stop 4b having a pair of openings juxtaposed, normal to the plane of the page, at a distance sufficient to ensure focusing accuracy; a pair of separator lenses 5b disposed, normal to the plane of tile page, behind the aperture stop 4b having a pair of openings; and light receiving element arrays 7 placed at imaging positions of light beams emerging from the separator lenses 5b. These optical components constitute the focus detecting device.
In the focus detecting device of Publication No. Sho 63-264715, two juxtaposed focus detecting systems are respectively taken as the focus detecting system I and the focus detecting system II. When the amount of phase difference of the images on the light receiving element arrays is represented by P, the F number of the barycentric beam to be detected is represented by F.sub.w, and the projecting magnification of one focus detecting system is denoted by .beta., the amount of defocus D to be detected is given by D=(F.sub.w /.beta.)P. Consequently, the amounts of defocus detected by the focus detecting systems I and II are expressed by EQU D.sub.1 =(F.sub.W1 /.beta..sub.1) P.sub.1 ( 5) EQU D.sub.2 =(F.sub.W2 /.beta..sub.2) P.sub.2 ( 6)
Even in the case where the light receiving element arrays are arranged on the same plane, this focus detecting device is designed to be able to set arbitrarily the spaces between the condenser lens and the separator lenses, and between the separator lenses and the light receiving element arrays so that the projecting magnifications .beta..sub.1 and .sub.62 .sub.2 of the focus detecting systems are made different from each other. Consequently, the focus detecting device is constructed with the focus detecting system I which is somewhat low in focusing accuracy but large in focus detecting area, and the focus detecting system II which is smaller in focus detecting area but higher in focusing accuracy. The focus detecting device can thus be brought about in which the extension of the focus detecting area and the improvement of the focusing accuracy are compatible with each other.
Also, Japanese Publication No. Hei 4-240813 proposes to increase the amount of light incident on the light receiving element arrays by switching over the optical path to another.
As explained in connection with FIGS. 1 to 8, the focus detecting device can be made to satisfy both the extension of the focus detecting area and the improvement of the focusing accuracy. In the case of the focus detecting device including the aperture stop of configuration shown in FIG. 5, however, its opening section cannot be made larger. This results in the reduction of the amount of light reaching the light receiving element arrays and the deterioration of the focusing accuracy. Specifically, If the opening section of the focus detecting system I is made larger, that of the focus detecting system II must be made smaller. For increasing the amount of light reaching the light receiving element arrays, It is only necessary to construct the aperture stop 4 of configuration shown In FIG. 9. The use of such an aperture stop, however, leads to a small difference between the F number F.sub.W1 of the barycentric beam to be detected in the focus detecting system I and the F number F.sub.W2 of that in the focus detecting system II. This defeats the primary purpose of satisfying both the extension of the focus detecting area and the improvement of the focusing accuracy.
As shown In FIG. 8, even though light is split up by the half mirror to travel along two focus detecting systems, the amount of light reaching the light receiving element arrays will decrease. Further, the arrangement, set forth in Publication No. Hei 4-240813, for increasing the amount of light incident on the light receiving element arrays by switching over the optical path needs moving members and the resultant space and driving power. Moreover, the application of these means to the focus detecting device causes increase of the total area of light receiving element arrays, followed by reduction of the workability of light receiving elements and bulkiness of the entire focus detecting system. Thus, this arrangement affects the compactness of the entire camera. The focus detecting device of the type has many problems such that adjustments are required for the half mirror dividing light to follow the focus detecting systems I and II, the path switching members, and Individual parts of the focus detecting systems I and II; the mechanism is complicated; and the assembly is troublesome.
In the conventional focus detecting device in which a pair of openings (entrance pupils) is disposed to be nearly equidistant from the optical axis of the condenser lens, If the reimaging lenses of identical shape are provided to be nearly equidistant from the optical axis of the condenser lens, particularly significant light beams from an object adjacent to the optical axis will likewise undergo the same refraction. Although the refraction changes progressively In separating from the optical axis if the extent of this change is made symmetrical, the focusing of the images in an in-focus state can easily be improved.
However, some camera systems need an arrangement such that the entrance pupils of the focus detecting systems are asymmetrical with respect to the optical axis of the photographic lens. One of its specific examples will be given below.
In FIGS. 10A and 10B, reference numeral 11 denotes a finder section; 12 a photographic lens section; 13 a focus detecting device section; 14 a screen mat; 15 a mirror box; 16 an imaging plane; 17 a main mirror; and 18 a field stop. Reference symbol BM represents a submirror. Where the entrance pupils of the focus detecting systems are arranged vertically in order to improve the focusing accuracy of an object in a horizontal direction, as shown in FIG. 10A, it is necessary to extend the field stop 18 in a lateral direction of the plane of the figure. Further, in order to provide the sub-mirror BM for introducing a beam of light into the field stop 18, there is the necessity of enlarging the mirror box 15.
On the other hand, FIG. 10B illustrates the case where the entrance pupils are arranged horizontally and the field stop 18 need not be extended. If the mirror box 15 is enlarged, not only does the entire camera body become larger, but the last surface of tile photographic lens must be located farther away from the imaging plane 16. This causes problems such as the oversizing of the photographic lens, the complication of the arrangement, and the degradation of imaging performance. Furthermore, there is a systematic problem of defeating the mounting of the photographic lens in which the distance between the last surface of the photographic lens and the imaging plane 16, although enough for a common camera body, is insufficient for the above camera system. Consequently, in the conventional camera system in which the entrance pupils of the focus detecting systems are arranged vertically, provision has been made to prevent the enlargement of the mirror box 15 in such a way as to diminish the NA of the light beam entering the focus detecting systems through the photographic lens, the focus detecting field, or the amount of detectable defocus. If, however, the entrance pupils of the focus detecting systems are arranged to be asymmetrical about the optical axis of the photographic lens, it will be possible to ensure the NA of the light beam from the photographic lens, the focus detecting field, and the amount of detectable defocus, without enlarging the mirror box 15.
In the optical system Including the condenser lens and the reimaging lens which have the same power, if the decentering relation between these two lenses is different from that between the opening section and the condenser lens, the conditions of incidence and emergence of light beams from the object will change. Thus, the effects of the reimaging system on the light beams vary, with the result that the coincidence between the images becomes incomplete even in the in-focus state. That is, the detecting accuracy of the phase difference degrades. Specifically, the images formed on individual light receiving element arrays in the same plane vary in size. The disagreement between tile images caused by the differences of distortion (aberration) of the images is corrected even in the conventional focus detecting optical system, as described later.
Next, reference is made to the functions of optical elements of the conventional focus detecting optical system and its degree of freedom for correction. The principal functions of the condenser lens in the conventional focus detecting optical system are (1) to perform the pupil transmission in the so-called relay optical system whose focal length is determined by the specification, and (2) to correct the distortion of the images on individual light receiving element arrays. The configuration of the condenser lens is practically determined by these two functions, and the number of degrees of other freedom is highly limited. On the other hand, the function of the reimaging lens in the focus detecting optical system is to determine where the image formed once at the preset imaging plane should be re-formed, and in the case of a simple optical system, the number of degrees of other freedom is highly limited. It is not easy that the above functions is added to individual optical elements so that the function of coincidence of two images is further added, that is, the images on the light receiving element arrays are made to coincide in size with each other. Further, if the number of lens elements is increased to complicate the arrangement, there will be the possibility that the coincidence of two images is improved, but error factors will be increased. This is unfavorable in view of assembly tolerance and adjusting means in particular.